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The ‘free’ ions needed to balance the ‘fixed’, covalently bonded ions in a conventional polyelectrolyte are often thought to play little role beyond charge balancing, processability and morphology control. In self-compensated, hole-doped polymers, free cations, together with mobile holes on the polymer backbone, balance the covalently tethered counter-anions. We show here that these ‘spectator’ cations not only influence the ionization energy (and work function) of the polymer, but also, perhaps surprisingly, determine the stability of the ultrahigh-workfunction state. We attribute these two effects to the dependence on the spectator cation size of surface ionic layering, and coulombic stabilization of the anion sites, respectively. The smaller cations provide larger coulombic stabilization of the anionic site, raising the energetic barrier for hole transfer to the anion or its hydration water, thereby blocking de-doping of the polymer backbone. We demonstrate these effects in an important model of a high-ionization-energy triarylamine–fluorene copolymer TFB with trifluoromethanesulfonylimidosulfonyl as a tethered counter-anion, and a family of spherical monovalent cations as spectators (Li+, Na+, Cs+, NMe4+ and NEt4+). Using Li+ as a spectator cation, we further demonstrate ultrahigh-workfunction hole injection layers for TFB semiconductor with markedly enhanced stability to ambient processing and baking. This work leads to a new design rule for stable, self-compensated, hole-doped polymer systems with ultrahigh work function.

Crosslinking of semiconductors is critical towards building heterostructures to enable flexibility in the design of devices. Bis(fluorophenyl azide)s (FPAs) and their sterically-substituted derivatives (sFPAs) have been demonstrated to be able to photo-crosslink semiconductor polymers upon deep-UV exposure, stabilizing their morphologies in devices. Here, we report the development of both UV (254 nm) and i-line (365 nm) photo-activatable FPA and sFPA – FPA80 and sFPA82 – both incorporating 3,5-bis(methylene)-1,1-dimethyl-4-piperidonium, a conjugation-extending bridge that also bears an ionic group. These materials are useful for generic photo-crosslinking of both conjugated and non-conjugated polyelectrolytes. Good thermal stability and solubility in polar organic solvents are obtained. Good film retention (>80%) can be achieved with a crosslinker loading of less than 10 wt/wt% for high-molecular polymers (Mw > 100k), superior to alkyl bisazides. The photo-crosslinking efficiency of ca. 20% in model polyelectrolytes is limited by the occlusion of ionic crosslinkers from the polymer matrix where the C–H insertion reaction occurs, hence suppressing photo-crosslinking efficiency.

Hole- and electron-doped polyelectrolytes are of much current interest because they afford workfunction-tuneable charge injection layers that are immune to ‘dopant’ migration. However the role of the covalently tethered ion beyond providing solubility in these systems have not been well understood. Here using two well-known fluorene-alt-triarylamine model semiconductor cores (TFB and mTFF), we show, using variable-angle X-ray and ultraviolet photoemission spectroscopies, that the larger tethered anions—trifluoromethanesulfonylimidosulfonyl and pentafluoroethanesulfonylimidosulfonyl—give stronger surface ionic layering than sulfonate. This contributes to increasing both the HOMO ionization energy of the undoped polymer, and the vacuum work function (WF) of the hole-doped polymer, by about 0.2 eV, which provides the desired ‘fine tuning’ of these parameters. In addition, molecular dynamic simulations of the ion cluster structure in the bulk show that the larger anions produce a larger average distance to the holes. This raises the electrochemical energy of the holes, advantageously increasing their inherent WF by another 0.1 eV or so. Device electroabsorption spectroscopy shows that the increased WF persists into devices, resulting in better hole injection efficiency into large-ionization-energy organic semiconductor layers, and better hole collection efficiency from organic solar cells. Finally, the larger tethered anions also impart better solubility to the hole-doped form in fluorinated alcohols, acetonitrile and propylene carbonate, overcoming a key bottleneck for the deployment of these materials in practical applications.

It is widely thought that the water-oxidation reaction limits the maximum work function to about 5.25 eV for hole-doped semiconductors exposed to the ambient, constrained by the oxidation potential of air-saturated water. Here, we show that polymer organic semiconductors, when hole-doped, can show work functions up to 5.9 eV, and yet remain stable in the ambient. We further show that de-doping of the polymer is not determined by the oxidation of bulk water, as previously thought, due to its general absence, but by the counter-balancing anion and its ubiquitously hydrated complexes. The effective donor levels of these species, representing the edge of the ‘chemical’ density of states, can be depressed to about 6.0 eV below vacuum level. This can be achieved by raising the oxidation potential for hydronium generation, using large super-acid anions that are themselves also stable against oxidation. In this way, we demonstrate that poly(fluorene-alt-triarylamine) derivatives with tethered perfluoroalkyl-sulfonylimidosulfonyl anions can provide ambient solution-processability directly in the ultrahigh-workfunction hole-doped state to give films with good thermal stability. These results lay the path for design of soft materials for battery, bio-electronic and thermoelectric applications., Realizing ultra-high work functions (UHWFs) in hole-doped polymer semiconductors remains a challenge due to water-oxidation reactions. Here, the authors determine the role of water-anion complexes in limiting the work function and develop a design strategy for realizing UHWF polymers.

Electrodes with low work functions are required to efficiently inject electrons into semiconductor devices. However, when the work function drops below about 4 electronvolts, the electrode suffers oxidation in air, which prevents its fabrication in ambient conditions. Here we show that multivalent anions such as oxalate, carbonate and sulfite can act as powerful latent electron donors when dispersed as small ion clusters in a matrix, while retaining their ability to be processed in solution in ambient conditions. The anions in these clusters can even n-dope the semiconductor core of π-conjugated polyelectrolytes that have low electron affinities, through a ground-state doping mechanism that is further amplified by a hole-sensitized or photosensitized mechanism in the device. A theoretical analysis of donor levels of these anions reveals that they are favourably upshifted from ionic lattices by a decrease in the Coulomb stabilization of small ion clusters, and by irreversibility effects. We attain an ultralow effective work function of 2.4 electronvolts with the polyfluorene core. We realize high-performance, solution-processed, white-light-emitting diodes and organic solar cells using polymer electron injection layers with these universal anion donors, demonstrating a general approach to chemically designed and ambient-processed Ohmic electron contacts for semiconductor devices. Multivalent anions are found to be capable of electron-doping polymer semiconductors to realize conductive films with very low work functions, which enable efficient electron injection into materials with low electron affinity.